I want a tiny, efficient, cheap class-D amp for active speakers. Both for hifi and for a portable battery powered setup.
I've read the thread about the chinese tpa3116 boards. While they are cheap, they look like they all suffer from bad board layout and worse components.
I'd like to build one that satisfies these requirements:
- High efficiency both at no output and when playing music with low crest factor at full power into 8Ohm.
- Low distortion.
- Doesn't radiate like a WWII radio jammer.
- Is cheap to build.

For active speakers it would probably be a good idea to have signal sensing turn on/off with timer output. For example a NJU7181 chip have everything you need integrated at an extremely low power consumption. Drive the SDZ (and FAULT) pin directly with the output, ie. no connection directly to Vcc. There are other similar chips but this one provides everything that is needed in one simple package and it costs about $0.50 each.

On another issue. Drop the LC output filter and have just a ferrite bead plus CRC output snubber. The main culprit of RF noise is long cables and the output inductors. Active speakers by definition have short speaker cables.

For active speakers it would probably be a good idea to have signal sensing turn on/off with timer output. For example a NJU7181 chip have everything you need integrated at an extremely low power consumption. Drive the SDZ (and FAULT) pin directly with the output, ie. no connection directly to Vcc. There are other similar chips but this one provides everything that is needed in one simple package and it costs about $0.50 each.

Note that there's probably no need for sensing 2hz signals so the 10µF input capacitor could just as easily be 220nF instead.

Also note you need a voltage divider or some other voltage regulator to supply the chip.

That is a good idea. I'll see if can squeeze that in.

WRT the output LC filters, I definitely wouldn't mind getting rid of the inductors. I just fear the efficiency would suffer from using the speaker as filter. I can't find any numbers though. Also, what about the harmonics and stuff from 400kHz up to where the ferrite starts working?

WRT the output LC filters, I definitely wouldn't mind getting rid of the inductors. I just fear the efficiency would suffer from using the speaker as filter. I can't find any numbers though. Also, what about the harmonics and stuff from 400kHz up to where the ferrite starts working?

You mean the AM frequency band? That's what the modulation is specifically designed to suppress so no problems there, it's higher order harmonics that can cause problems so a ferrite bead is used to suppress that.

Efficiency penalty of a ferrite bead is in the range of 0.3% to 2.1% depending on the speaker inductance.

The ferrite beads i've looked at really don't do much up to about 30MHz with any sort of DC bias. That makes me slightly uncomfortable.

That is energy wasted in the ferrite bead or the speaker?

They don't need to. The FM radio band is 100MHz +-10%, so the ferrite beads needs to have their impedance maximum there. Fortunately, that's also what the vast majority of ferrite beads is designed for.

I cannot stress enough that in my opinion the audio quality is a lot better with ferrite beads instead of LC output filter. The bass and lower midrange is a lot better detailed and controlled but it's the very high range that really stands apart. In that regard it's like the difference between a (high quality) .mp3 and a .wav file.

Lower efficiency means a higher percentage is lost in the amp heat sink but you have almost zero loss in the ferrite beads themselves due to the high frequency impedance resonance and the ultra low series resistance. They do not get even remotely warm. Output inductors on the other hand can get burning hot with added loss and even lower audio quality as a consequence due to thermally variable resistance and inductance as well as saturation in some cases.

I've removed the LC filters and replaced it with some beefier ferrite beads. Then i took a look at the eval board and realized that it was much nicer than what i was doing, so i changed the output routing completely. And i copied their RC-snubbers.

Any idea if it's going to behave?

I'm not really sure what i'm looking for with the ferrite beads, apart from a DC bias of up to 5A. The ones i've found ( http://datasheet.octopart.com/HI3312...heet-65586.pdf ) look kind of large.
I'm pretty sure i've overdone it with the vias.
I'm feeling a bit out of my depth with this stuff.

Anyway, this layout did free up a lot of space for power management and whatnot. So there's that.